There is a standard. A power source only needs to provide sufficient voltage to reach it’s rated power, given the maximum current of 3A on a non-eMarked cable, given voltage steps of 5V, 9V, 15V, 20V. Offering higher voltages than is neccesary is optional. This means that only adapters greater in power than 27W are required to support 15V, and only adapters greater in power than 45W are required to support 20V.
IMO, this is not unresonable. Would you prefer manufacturers be forbidden from offering voltage profiles higher than is necessary for their power rating?
(This is not neccesarily true for powers above 60W, and ignores PPS, EPR, and AVS. Above 60W you need eMarked cables and it gets a bit more complicated)